The subject matter disclosed herein relates generally to diagnostic imaging systems, and more particularly to positron emission tomography (PET) imaging systems and geometric calibration of the PET imaging systems.
PET imaging systems typically generate images depicting the distribution of positron-emitting nuclides in patients based on coincidence emission events detected using a detector system, usually configured as a detector ring assembly of detector blocks. In PET image reconstruction, data corrections are implemented that account for the geometric properties of the detector system. For example, the angle between two detector faces affects the number of coincidence emissions measured. Additionally, the physical design and alignment of the detector blocks also include geometric variances, such as the location of a crystal within a detector block. Moreover, manufacturing tolerances affect the physical alignment of each PET system differently, including the PET detector crystals and photomultiplier tubes.
The differences in materials and manufacturing processes, thus, affect the geometric calibration measurement and resulting correction coefficients for these PET systems. If the system geometry is not accounted for during image reconstruction, then image artifacts can appear. These artifacts include “bands” (also referred to as “rings”). For example, bands in axial slice images appear as streaks in reformatted coronal and sagittal images. Additionally, inconsistencies at the image center, such as “divots”, can also occur.
Conventional methods measure and obtain corrections for these geometric variations. For example, some conventional calibration methods to perform geometric correction acquire a scan of a rotating pin source at the outside of the PET field of view (FOV), which allows measurement of coincidence pairs for every line of response (LOR) for the system. However, because the pin source is of a finite radius, acquisition of an ideal line source is prevented. Additionally, acquisition imperfections from detector dead time issues can affect the data. Also, because the pin rotates very close to the detector faces, the detectors near the pin can become overloaded with counts, increasing the dead time and degrading the quality of the resulting geometric calibration.